Actions

The actions bag passed to createAtom becomes part of the atom’s method surface. Two kinds of entries are supported.

Functions

Plain function values get bound to the atom instance, so this inside them refers to the atom.

const counter = createAtom({
  key: "counter",
  default: 0,
  actions: {
    increment() {
      this.update(this.value + 1);   // `this` is Atom<number, ...>
    },
    addAndReturn(n: number) {
      this.update(this.value + n);
      return this.value;
    },
  },
});

counter.increment();
const next = counter.addAndReturn(5);

Typing this explicitly

If TypeScript can’t infer this (rare in practice), spell it out:

actions: {
  toggle(this: Atom<boolean>) {
    this.update(!this.value);
  },
}

The ThisType<Atom<V, A>> in the options helps inference in most cases.

Property getters

A getter on the actions object becomes a getter on the atom. Useful for derived properties.

const cart = atomCollection<Item>({
  key: "cart",
  actions: {
    get total() {
      return this.value.reduce((sum, i) => sum + i.price * i.qty, 0);
    },
    get isEmpty() {
      return this.value.length === 0;
    },
  },
});

cart.total;    // recomputed on each read
cart.isEmpty;

Getters are forwarded as property getters on the atom — calling .bind on them (which the action-installation loop does for functions) would invoke the getter prematurely and crash. The implementation detects descriptors and routes them correctly.

Plain values

Non-function, non-getter values are copied through as-is:

actions: {
  MAX_SIZE: 100,
  DEFAULT_OPTIONS: { foo: true },
}

These show up as plain properties on the atom; modifying them at runtime is allowed but not observed by the event bus.

Patterns the React layer builds on

@mongez/react-atom uses this same actions mechanism to attach React-specific methods to every atom it creates:

• atom.useValue() — a hook returning the current value (via useSyncExternalStore).
• atom.useState() — [value, setValue] tuple.
• atom.use(key) — fine-grained subscription to one key.
• atom.useWatch(key, cb) — effect wrapper.
• atom.Provider — a React component that pushes a value into the atom on mount.

You can build the same pattern for your own framework — Vue composables, Solid signals, etc. The actions are just bound methods.